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POWER GENERATION AND TRANSMISSION
25
absolute back pressure, lower figures are for condensing machines at 28-in. vacuum.
Steam pressure, pounds	Superheat, degrees Fahrenheit					
per square inch						
absolute	0	40	80	120	160	200
	25.5	24.8	24.0	23.2	22.3	21.5
100						
	10.5	10.3	10.0	9.8	9.5	94.3
	23.2	22.5	21.8	21.2	20.4	19.6
120						
	10.1	9.9	9.6	9.4	9.3	9.0
	21.6	21.0	20.3	19.8	19.0	18.3
140						
	9.9	9.6	9.4	9.2	9.0	8.8
	20.3	19.7	19.1	18.6	18.0	17.3
160						
	9.6	9.4	9.2	8.9	8.7	8.5
	19.4	18.8	18.3	17.7	17.1	16.5
180						
	9.4	9.2	9.0	8.8	8.6	8.4
	18.7	18.1	17.6	•  17.2	16.5	16.0
200						
	9.3	9.0	8.8	8.6	8.4	8.3
	18.0	17.5	17.0	16.5	16.0	15.5
220						
	9.2	8.9	8.7	8.5	8.3	8.2
The steam consumptions of actual turbines exceed these values, for the following reasons:
1.  Nozzle friction, which may amount to 10 per cent.
2.  Residual velocity, not utilized in buckets, causing a loss which is reduced when wheel speeds are high or several stages are used.
3.  Incomplete utilization of velocity due to limitations of stream and bucket
4.  Bucket friction.
5.  Mechanical friction and windage.
6. Electrical losses, partly constant and partly varying with the load.
Item (2) is the loss which chiefly determines efficiency. If n = number of rows of buckets, d = pitch diameter, inches, r = revolutions per minute, the accompanying curve gives values of relative efficiency, Ei, plotted against the quantity nd2r2 -f-1,000,000. The actual steam rate will then be the appropriate value from the table above, divided by Ei, and by the electrical efficiency. Any large turbine of theCurtis or modern pressure type, with the usual superheat and a good vacuum, may be expected at rated load to use not over 16 Ib. of steam per kilowatt-hour. Non-condensing turbines and small turbines are inefficient, the latter in part because of the necessary simplicity of construction. High vacuum is particularly desirable in turbine installations. Superheat not only pays thermodynamically, but also decreases erosion and steam friction. Figure 19 shows the "flat" type of characteristic to be expected with a good machine. Turbines generally are differentiated from reciprocating engines by low friction losses, large potential capacities and high-rotative speeds.
Low-pressure Turbines.—The capacity of an existing reciprocating engine plant (even though operated condensing) may be increased from 60 to 100 per cent by the appropriate installation of a turbine receiving steam exhausted by